Taxonomy of Hibiscus is in flux. Molecular studies place members of some other genera in Hibiscus as traditionally circumscribed, indicating paraphyly (B. E. Pfeil and M. D. Crisp 2005). Neither of the two solutions to this problem—vastly expanding the circumscription of Hibiscus to include the nested genera, or breaking Hibiscus into smaller genera—has been attempted here.

Most Hibiscus species in the flora area are native; seven were introduced into North America either for horticultural purposes (H. acetosella, H. mutabilis, H. rosa-sinensis, H. schizopetalus, H. syriacus, and H. trionum) or as fiber crops (H. radiatus). Experimental crosses among some native species (H. coccineus, H. grandiflorus, H. laevis, and H. moscheutos) have been the basis of selections of hardy Hibiscus available in the horticultural trade. A popular cultivated form in the South is the result of an experimental cross involving a complex interspecific hybrid combining H. coccineus, H. laevis, and H. moscheutos as the staminate parent and H. mutabilis as the pistillate parent (H. F. Winters 1970).

North American taxa in the speciose sect. Bombicella share a base chromosome number of × = 11 (chromosome numbers are unknown for Hibiscus coulteri); those in sect. Furcaria, × = 18; and those in sect. Muenchhusia, × = 19. Base numbers of the two sections represented in the flora area by one native species each are 10 and 26, and cannot be readily reconciled between themselves or with the others. The remaining four sections together include five species, all of which are non-natives and, except for H. trionum, have long histories in cultivation and multiple different chromosome numbers that are not easily evaluated. This overall chromosomal diversity supports the molecular evidence for a polyphyletic Hibiscus.

Extensive experimental hybridization work in sect. Furcaria, and the study of chromosome pairing relationships in these hybrids, indicate that the native species Hibiscus aculeatus and H. furcellatus, both tetraploids, share the genomic makeup GGPP, while the introduced species H. acetosella and H. radiatus, also tetraploids, share AABB (F. D. Wilson 1994).

A naturally occurring hybrid between Hibiscus coulteri and H. denudatus (Hibiscus ×sabei Weckesser) has been documented from western Texas (W. Weckesser 2011).

Some desert species of Hibiscus sometimes produce cleistogamous flowers. Foliar nectaries are present abaxially near the base of the midvein in some species.

Three species of Hibiscus are of doubtful status in the flora area. Hibiscus cannabinus Linnaeus and H. sabdariffa Linnaeus have both been cultivated in the southern states as fiber crops, and the latter also for its fleshy calyces, which are used to flavor beverages. It is doubtful that either has become established in the flora area. Hibiscus bifurcatus Cavanilles is widely distributed from the West Indies and Guatemala to South America; it was documented with two collections from Dade County in Florida by R. Woodbury in 1949 and 1950, but since Woodbury was involved at the time in establishing what later became the John C. Gifford Arboretum in Coral Gables south of Miami, there is a good chance that the source was a plant in cultivation.